Electron beam generating devices, such as x-ray tubes and electron-beam welders, generally operate in high-temperature environments. During operation of an x-ray tube, for example, the primary electron beam generated by its cathode deposits a very large heat load on its anode target such that the target glows red-hot. Typically, less than 1% of the primary electron beam's energy is converted into x-rays, while the balance of its energy is converted into thermal energy. In general, this thermal energy from the hot anode target is radiated to various components within the x-ray tube's vacuum vessel and thereby causes the x-ray tube to heat up. Furthermore, some of the electrons in the electron beam backscatter from the anode target and impinge on these same components within the vacuum vessel, thereby causing additional thermal heating of the x-ray tube. As a result of the elevated temperatures caused by the cumulative effects of such thermal energies, the x-ray tube's components are subjected to high thermal stresses that are sometimes undesirable for proper operation of the x-ray tube itself.
Typically, an x-ray beam generating device, such as an x-ray tube, includes opposing electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel itself is typically fabricated from glass or a metal, such as stainless steel, copper, or a copper alloy. The electrodes themselves generally comprise a rotating, disc-shaped anode assembly and also a cathode assembly that is positioned at some distance from the target surface or track on the disc-shaped anode assembly. In other applications, the anode or anode assembly may alternatively be stationary. The target surface or track (or impact zone) of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or a tungsten alloy. To properly accelerate electrons toward the anode, a voltage potential difference of about 60 kilovolts (kV) to about 140 kV is typically maintained between the cathode and anode assemblies. In such a configuration, the cathode's hot filament emits electrons that are accelerated across the resultant electric field so that the electrons impact the target track of the rotating anode at high velocities. Typically, only a small fraction of the electrons' kinetic energies is converted into high-energy electromagnetic radiation or x-rays, while the balance of the energies is either retained in backscattered electrons or converted into heat. In general, the resultant x-rays emanate from the electron beam's focal spot on the anode and are therefrom directed out of the vacuum vessel. In an x-ray tube that particularly has a metal vacuum vessel, an x-ray transmissive window is fabricated and incorporated into the wall of the vacuum vessel so as to allow the x-ray beam to exit the vessel at a desired location. After exiting the vacuum vessel, the x-rays are directed so as to irradiate a particular object, such as a region of interest (ROI) within a human's anatomy for medical examination and diagnosis purposes. After the x-rays pass through the object, they are generally intercepted by an x-ray detector, from which an image is generated and formed of the anatomical ROI. Furthermore, in addition to such a medical application, x-ray tubes may alternatively be utilized in industry to, for example, inspect metal parts for cracks or inspect the contents of luggage at an airport.
As alluded to above, many of the electrons incident on the anode are not converted into x-rays and are instead backscattered from the anode's target surface in random directions. For example, up to about 50 percent of electrons incident on an anode target made of tungsten are typically backscattered. These backscattered electrons generally travel on a curvilinear path through the electric field between the cathode and anode until they impact one or more nearby structures or components. During such backscattering, these electrons interact with the electric field and space charge therein, thereby causing their initial trajectories to be altered in a complicated, but predictable, manner. As these backscattered electrons impact internal components of the x-ray tube, their kinetic energies are transferred to the components in the form of thermal energy until generally all of their respective energies are depleted. Furthermore, in addition to transferring thermal energy to the tube's internal components, the impact of backscattered electrons also produces additional x-ray radiation, termed “off-focal x-rays” in medical x-ray applications. In general, the production of such off-focal x-ray radiation tends to degrade x-ray imaging quality if it is allowed to exit the vacuum vessel's x-ray transmissive window.
The paths of backscattered electrons, and therefore the paths of off-focal radiation, can be influenced by the particular electric voltage potential configuration in and about the x-ray tube. In a bi-polar configuration, for example, the cathode is maintained at a negative potential, and the anode is maintained at a positive potential relative to electrical ground, thereby establishing a voltage potential drop and electric field across the gap between the cathode and the anode. In this configuration, a large fraction of electrons initially backscattered from the anode are drawn back to the anode by its electrostatic potential. On the other hand, in a uni-polar configuration, both the anode and vacuum vessel are electrically grounded, and the cathode is maintained at a high negative potential. In this uni-polar configuration, the attractive force of the electrically grounded anode and frame is less than the attractive force of a positively charged anode and frame of an x-ray tube in a bi-polar configuration. Therefore, in a uni-polar configuration, a larger fraction of backscattered electrons can generally be collected and not allowed to return to the anode, thereby significantly enhancing the operating performance of the anode and also decreasing the amount of off-focal x-ray radiation exiting through the transmissive window.
Since the production of x-rays in a conventional x-ray tube is somewhat inherently an energy-inefficient process, the various components within such an x-ray tube typically operate at very high temperatures. For example, the temperature of the anode's target surface during operation exceeds 2000° C. Furthermore, the temperature of much of the anode assembly exceeds 1000° C.
To help cool the x-ray tube, the thermal energy generated during tube operation is generally transferred from the anode and through the vacuum vessel so that it can be removed with a heat-absorbing cooling fluid. To accomplish such, the vacuum vessel itself is typically enclosed in an outer casing that is filled with a circulating cooling fluid such as, for example, a dielectric oil. In such a configuration, the casing further supports and protects the x-ray tube and also provides for attachment to, for example, the rotating gantry of a computed tomography (CT) imaging system. The casing itself may be lined with lead to help shield and prevent any extraneous x-ray radiation from straying from the tube. In general, the cooling fluid in the casing performs two duties. These duties include cooling the vacuum vessel and also providing high-voltage insulation between the anode and cathode connections when in the above-mentioned bi-polar configuration. During operation of the x-ray tube, however, the performance of the cooling fluid may be degraded over time by excessively high temperatures that cause the fluid to boil at the interface between the fluid and the outer surface of the vacuum vessel or vacuum vessel's transmissive window. When the cooling fluid is caused to boil in this manner, large bubbles may form within the fluid that undesirably facilitate high-voltage arcing across the fluid, thus degrading the insulating capability of the fluid. Furthermore, the bubbles may give rise to x-ray image artifacts that produce low-quality images.
In addition to facilitating arcing, excessively high temperatures in an x-ray tube can also decrease the useful life of the tube's transmissive window, as well as other tube components. Because of its conventionally close proximity to an electron beam's focal spot on the anode's target surface during tube operation, the x-ray transmissive window is subjected to very high heat loads resulting from thermal radiation and backscattered electrons. Such high thermal loads on the transmissive window generally necessitate careful tube design to ensure that the window operates properly over the life of the x-ray tube, especially for the purpose of helping maintain a vacuum in the tube's vessel as the transmissive window is an important part the x-ray tube's overall hermetic seal. In general, the high heat loads in an x-ray tube cause very large and cyclic stresses in the transmissive window and can lead to premature failure of the window and its hermetic seal(s). Furthermore, since direct contact of the window (when excessively hot) with the cooling fluid can cause the fluid to boil as it flows over the window, degraded hydrocarbons from the fluid are sometimes apt to deposit on the window's outer surface, which can undesirably reduce x-ray imaging quality.
In view of the above, there is a present need in the art for a system or structure that effectively collects backscattered electrons within an x-ray tube's vacuum vessel and that also effectively transfers thermal energy attributable to such collected electrons from the tube.